A refrigerator or an air conditioner is installed with a valve drive device in which a valve element is moved by a drive source such as a stepping motor to open and close a flow path for refrigerant to control temperature in a chamber which is in communication with the flow path. Further, a methanol type of fuel cell has been recently developed as a power supply for a mobile apparatus such as a notebook type of personal computer. A valve drive device is also utilized to adjust a flow rate of fuel when the fuel (methanol and water) as a reducing agent is supplied to a fuel electrode layer (cell) in the methanol type of fuel cell.
As described above, a valve drive device is utilized in various fields. In this case, as an apparatus on which the valve drive device is mounted is downsized, it becomes more and more important to accurately control the flow rate of fluid such as refrigerant or fuel. One of measures for accurately controlling the flow rate of fluid is to improve the closing performance (sealing property) of a valve element (see, for example, Japanese Patent Laid-Open No. 2005-54954 and Japanese Patent Laid-Open No. 2004-327775).
In the above-mentioned former patent reference, a control valve is disclosed in which a sealing property in a closing state of a valve element is improved. Specifically, in the former reference, a valve drive device is disclosed in which a drive shaft of a stepping motor is reversibly rotated to linearly move an elevating body up and down and the step-out of the motor is detected with an encoder to control an opening degree of a diaphragm (control valve). In the valve drive device, a projection is formed at a periphery of an inflow opening of fluid and the projection is abutted with a control valve. Therefore, the sealing property in the closing state of the valve element is improved.
Further, a substrate processing apparatus and a discharge control method in which discharging accuracy from a nozzle for drug solution is improved is disclosed in the above-mentioned latter patent reference. Specifically, in the latter reference, a substrate processing apparatus is disclosed which includes a flow control valve for opening or closing a pipe through which drug solution is circulated and an encoder. In the substrate processing apparatus, the step-out of a motor is detected with the encoder to cause the flow control valve to be in a closing state or to cause a moving quantity (stroke) of the flow control valve to be adjusted according to property of the drug solution and thus discharging accuracy from a nozzle for drug solution is improved.
As described above, in the above-mentioned patent references, the step-out of a motor is detected with an encoder to close a flow path with a control valve, and the control valve is abutted with a projection or the stroke of the control valve is adjusted to improve the sealing property with the valve element.
However, in the above-mentioned patent references, as described above, the step-out of a motor is detected with an encoder and the closing state of a valve is maintained by using a detent torque of the motor. Therefore, since error of the motor and errors of respective members from a rotor to a valve element are accumulated or piled up, a stable sealing property in the closing state of the valve element is not obtained.
For example, in a state that a valve element of a control valve or the like abuts with an inflow opening (or outflow opening) for fluid to close a flow path, since the motor is stepped out, a closing force for closing the inflow opening (or outflow opening) varies in a range of a stroke length of elastic deformation of the valve element and in a range of the number of detent poles of the motor to cause the closing force to vary whenever the flow path is closed. As a result, since closing performance of the valve element is varied, the sealing property in the closing state of the valve element is not stable.
Further, when the valve element is pressed with too strong force at the time when the flow path is closed, an elastic force of the valve element is varied due to hysteresis or the abutting part is worn. As a result, the closing performance of the valve element is further varied and the sealing property in the closing state of the valve element becomes further more unstable.
As described above, in the conventional valve drive device, since the sealing property in the closing state of the valve element is hard to stabilize, reliability for closing performance of the valve element is not satisfactory.